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Design Patterns by Tutorials

Third Edition · iOS 13 · Swift 5 · Xcode 11

Before You Begin

Section 0: 4 chapters
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Section I: Hello, Design Patterns!

Section 1: 2 chapters
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Section II: Fundamental Design Patterns

Section 2: 7 chapters
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Section III: Intermediate Design Patterns

Section 3: 8 chapters
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Section IV: Advanced Design Patterns

Section 4: 7 chapters
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7. Memento Pattern
Written by Joshua Greene

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The memento pattern allows an object to be saved and restored. It has three parts:

  1. The originator is the object to be saved or restored.

  2. The memento represents a stored state.

  3. The caretaker requests a save from the originator and receives a memento in response. The caretaker is responsible for persisting the memento and, later on, providing the memento back to the originator to restore the originator’s state.

While not strictly required, iOS apps typically use an Encoder to encode an originator’s state into a memento, and a Decoder to decode a memento back to an originator. This allows encoding and decoding logic to be reused across originators. For example, JSONEncoder and JSONDecoder allow an object to be encoded into and decoded from JSON data respectively.

When should you use it?

Use the memento pattern whenever you want to save and later restore an object’s state.

For example, you can use this pattern to implement a save game system, where the originator is the game state (such as level, health, number of lives, etc), the memento is saved data, and the caretaker is the gaming system.

You can also persist an array of mementos, representing a stack of previous states. You can use this to implement features such as undo/redo stacks in IDEs or graphics software.

Playground example

Open FundamentalDesignPattern.xcworkspace in the Starter directory, or continue from your own playground workspace from the last chapter, and then open the Overview page.

What should you be careful about?

Be careful when adding or removing Codable properties: both encoding and decoding can throw an error. If you force unwrap these calls using try! and you’re missing any required data, your app will crash!

Tutorial project

You’ll continue the RabbleWabble app from the previous chapter.

Key points

You learned about the memento pattern in this chapter. Here are its key points:

6. Singleton Pattern
8. Observer Pattern
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